Accelerator-based neutron sources have many potential applications, including medical treatments, isotope production, explosive/fissile materials detection, assaying of precious metal ores, imaging, and others. A particular area of interest is boron neutron capture therapy (BNCT), which is a cancer treatment technique in which boron is preferentially concentrated in a patient's malignant tumor and a neutron beam is aimed through the patient at the boron-containing tumor. When the boron atoms capture a neutron, particles are produced having sufficient energy to cause severe damage to the tissue in which it is present. The effect is highly localized, and, as a result, this technique can be used as a highly selective cancer treatment method, effecting only specifically targeted cells.
One of the most commonly proposed neutron target materials for these types of systems is lithium, which reacts upon treatment with protons to produce neutrons through the reaction 7Li(p,n)7Be. This reaction has a high neutron yield and produces neutrons of modest energy, desirable for many applications.
However, since the energy of the proton beam is dissipated as heat in the target, the heat must be removed before the target is destroyed. Two primary approaches have been proposed for heat removal. The first is a stationary solid target, intensively cooled, mainly through water cooling, from the backside. The second is a liquid target in which the proton beam impinges on a flowing jet of liquid source material. Both of these approaches have significant drawbacks, particularly when lithium is used as the neutron source/target. Lithium has a relatively low melting temperature (180° C.) and a relatively low thermal conductivity, which makes it very challenging to remove the heat from a solid target without overheating and melting the surface. In addition, exposure to intense proton beams can quickly lead to blistering of the solid lithium, requiring frequent target replacement. Furthermore, lithium is highly reactive with water, so a water cooling system can be problematic if a malfunction occurs.
While liquid target solutions have been described, these, in general, suffer from slow heat-up times and potential solidification of flowing lithium if the temperature in the circuit drops too low, causing the charge of lithium to be inadvertently diverted into the target chamber. Flowing liquid lithium approaches also require a large amount of lithium to fill up the circuit, pump, and heat exchanger, which leads to both high cost and a significant safety hazard from the highly reactive liquid lithium.